Of the various electrostatic printing methods, electrophotography has dominated high resolution monochrome printing for several decades. The electrophotographic process includes uniformly coating a photoconductive surface with charge, selectively exposing the charged surface with light to form a latent image, developing the latent image by causing charged toner particles to come in contact with it, transferring the image to a receiving sheet, and fixing the image. This printing method has produced high quality printing and has been refined to effectively service a broad range of printing applications. However it is mechanically complex, requires precision optical components, and has proven difficult to adapt to color printing.
Direct Electrostatic Printing (DEP) can be simpler than electrophotographic printing. In U.S. Pat. No. 3,689,935 Pressman et al disclose a DEP device in which toner is deposited directly through apertures onto a plain paper substrate in image configuration. This method has been improved by Schmidlin in U.S. Pat. No. 4,912,489 issued Mar. 27, 1990 in which a control voltage as low as 100 V is sufficient to modulate the flow of toner through the apertures. The Schmidlin device employs a travelling wave conveyor to present toner to the printhead apertures. Generally, DEP processes have used single component insulating toners, and this has helped to simplify printing systems compared with prior two-component toner systems. The DEP devices do not require optics for the exposure step, nor do they require a photoconductive surface. Thus the printing process has been simplified. However, the apertures in the prior art DEP printers discussed above are subject to clogging arising from toner agglomeration. Additionally, ambient dust may clog an aperture. In either case, repair of a clogged aperture is difficult and costly. DEP line printers generally use multiple lines of apertures, for example four lines. The multiple lines of apertures are the result of manufacturing considerations relating to the apertures which are formed in a supporting member, with space provided for metal electrodes surrounding each electrode. This arrangement requires separating a pixel line of monochrome image data into data subsets, each subset corresponding to a line of apertures and their associated electrodes. Each data subset must be printed at a different time, with paper advances in between, in order that the original pixel line is reconstructed as a single line on the paper. Color printing generally requires the superposition of multiple monochrome images, and is more complex.
Travelling wave devices have been used to move particles along a tubular duct of insulating material. U.S. Pat. No. 3,778,678 issued to Masuda describes such a device which has three elongated electrodes spirally wound along its outer surface, uniformly spaced from one another. The electrodes are connected with the terminals of an alternating current source having a voltage of the order of 5-10 kV to produce a wave-like electric field within the duct by which particles are repelled from the inner duct surface and repulsively propelled along the duct. It is an object of the Masuda device to levitate the particles near the center of the tube, so that they do not make contact with the tube walls. A similar Masuda device is disclosed in U.S. Pat. No. 3,872,361 which discloses annular electrodes as well as elongated electrodes.
In U.S. Pat. No. 4,743,926 Schmidlin et al disclose a toner/developer delivery system that includes a pair of charged toner conveyors which are supported in face-to-face relation. A bias voltage is applied across the two conveyors to cause toner of one charge polarity to be attracted to one of the conveyors while toner of the opposite polarity is attracted to the other conveyor. Another embodiment includes a single charged toner conveyor supplied by a pair of three-phase AC current generators which are biased by a DC source which causes toner of one polarity to travel in one direction on the electrode array while toner of the opposite polarity travels generally in the opposite direction. In U.S. Pat. No. 4,876,561, also issued to Schmidlin, the charged toner conveyor may have over 400 electrodes per inch to enable a high toner delivery rate without risk of air breakdown.
U.S. Pat. No. 4,491,855 issued to Fujii at al, discloses an improved device for delivering charged particles to the vicinity of imaging electrodes. The improvement lies in that the charged particles are supported on a supporting member and an alternating electric field is applied between the supporting member and the control electrode. In U.S. Pat. No. 4,568,955 Hosoya et al disclose a recording apparatus using a toner-fog generated by electric fields applied to electrodes on the surface of a developer carrier. The electric fields are produced by an AC and a DC source connected to the electrodes, causing oscillations of the developer which generates the toner fog. U.S. Pat. No. 4,653,426 issued to Kohyama discloses the improvement of an AC voltage whose frequency varies with time. The voltage is applied to the gap between a toner carrying surface and a drum whose surface contains an electrostatic latent image. The multiple frequencies increase the toner particle jumping probabilities, thus improving gradation and denseness properties of the resulting developed image.
U.S. Pat. No. 4,876,575 issued to Hays discloses an apparatus for dynamic toner metering and charging of nonmagnetic single component toner. The apparatus includes a flexible rotating rod having an electric bias applied thereto, and the rod is in a self-spaced relationship to a rigid donor roll. The apparatus effectively creates a monolayer of charged toner on the donor roll.
The author's co-pending application, Ser. No. 07/658,397 filed Feb. 20, 1991 is incorporated herein by reference. It describes multiphase clocks applied to a linear array of electrodes to implement an imaging conveyor; toner delivered to each pixel site on the receiving sheet can be individually controlled by application of voltages to the electrodes. However, the imaging conveyor in the co-pending application uses dense multilayer thin film circuits which may be expensive to manufacture. By comparison, the present invention requires only a single toner conveyor for full color printing, most of the thin film circuitry is single layer, and the thin film circuits are less dense for a given print resolution. Furthermore, only one integrated circuit driver is required for each pixel site.